1. Field of the Invention
The present invention relates to a variable focal length optical system and, more particularly, to a compact variable focal length optical system which can achieve a high zoom ratio and whose total lens length in a maximum telephoto state (largest focal length state) is small.
2. Related Background Art
In recent years, cameras each having a zoom lens have become popular as lens shutter type cameras. Particularly, cameras each having a so-called high variable magnification zoom lens with a zoom ratio of 3.times. or more have become popular.
For a lens shutter type camera, even when the zoom power ratio of a taking lens is high, the camera is required to be compact and lightweight. Therefore, it is important to develop a lens system suitable for reducing the size of the lens system or the total length of the lens.
Unlike a single-lens reflex camera, the lens shutter type camera has no limitation in the back focus of the taking lens system. Therefore, in many taking lens systems, a negative lens unit is arranged in the lens system to be closest to the image, thereby reducing the lens diameter or the total lens length. In a maximum wide-angle state (smallest focal length state), when the back focus is decreased, and an off-axis light flux passing through the negative lens unit is separated from the optical axis in accordance with a change in field angle, an on-axis aberration and an off-axis aberration are independently corrected. The back focus is increased when an operation for changing the state of lens positions from the maximum wide-angle state to the maximum telephoto state is performed. With this arrangement, the height of the off-axis light flux passing through the negative lens unit is changed in accordance with the operation for changing the state of lens positions to suppress a variation in off-axis aberration caused by the operation for changing the state of lens positions, thereby obtaining satisfactory imaging performance.
Conventionally, as a zoom lens having a zoom ratio of about 2.times., a positive/negative two-unit type zoom lens is popularly used, which comprises a positive lens unit and a negative lens unit in this order from the object side and moves each lens unit to change the distance between the positive lens unit and the negative lens unit in an operation for changing the state of lens positions.
In this positive/negative two-unit type zoom lens, only the second lens unit has a large change in lateral magnification with respect to a change in focal length of the entire lens system when the lens position state changes from the maximum wide-angle state to the maximum telephoto state. When a zoom ratio of 2.times. or more is to be realized, a change in lateral magnification of the second lens unit, which is caused by the operation for changing the state of lens positions, becomes large. Therefore, the variation in off-axis aberration generated in the operation for changing the state of lens positions cannot be satisfactorily corrected.
In the maximum wide-angle state, the refractive power arrangement is a large asymmetry. To correct a positive distortion, the first lens unit is constituted by a negative lens and a positive lens. Consequently, the total lens length can hardly be decreased in the maximum telephoto state.
Therefore, various proposals have been made for a zoom lens which uses a so-called multi-unit zoom lens constituted by three or more movable lenses to decrease a large change in lateral magnification of each lens unit with respect to a change in focal length of the entire lens system when the lens position state changes from the maximum wide-angle state to the maximum telephoto state and to achieve a high zoom ratio or size reduction. These proposals are disclosed in, e.g., Japanese Laid-Open Patent Application No. 6-265788 and Japanese Laid-Open Patent Application No. 7-27979 filed by the present applicant.
The zoom lens disclosed in Japanese Laid-Open Patent Application No. 6-265788 comprises, in the following order from the object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a negative refractive power. The zoom lens disclosed in Japanese Laid-Open Patent Application No. 7-27979 comprises, in the following order from the object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a positive refractive power, and a fifth lens unit having a negative refractive power. In both the lens systems, a negative lens unit is arranged to be closest to the image, and all lens units move to the object side in the operation for changing the state of lens positions from the maximum wide-angle state to the maximum telephoto state.
In the zoom lens disclosed in Japanese Laid-Open Patent Application No. 6-265788, however, the fourth lens unit has a small change in lateral magnification with respect to a change in focal length of the entire lens system when the lens position state changes from the maximum wide-angle state to the maximum telephoto state while the second lens unit has a very large change in lateral magnification with respect to a change in focal length of the entire lens system when the lens position state changes from the maximum wide-angle state to the maximum telephoto state. As a result, the angle of incidence of the off-axis light flux passing through the second lens unit largely changes in zooming although the height of the off-axis light flux merely changes. Therefore, a variation in off-axis aberration generated in the operation for changing the state of lens positions can hardly be suppressed.
When a lot of aspherical surfaces are used, an aberration can be corrected within a limited focal length range such as in the maximum wide-angle state, the intermediate focal length state, or the maximum telephoto state. However, a variation in off-axis aberration can hardly be suppressed within the entire focal length range from the maximum wide-angle state to the maximum telephoto state.
From another viewpoint, though the technique of manufacturing an aspherical lens is advanced today, the performance may be largely degraded by an eccentricity generated in manufacturing when too many aspherical lenses are used. In addition, the contrast of a high spatial frequency component is lowered by an error in plane precision with respect to the design value, which is generated in manufacturing. Therefore, the zoom lens disclosed in Japanese Laid-Open Patent Application No. 6-265788 cannot sufficiently satisfy high performance.
In the zoom lens disclosed in Japanese Laid-Open Patent Application No. 7-27979, the focal length of the first lens unit is large relative to that of the entire system in the maximum telephoto state, and the converging action is small. Therefore, this zoom lens is not so suitable for reducing the total lens length.